It's one of the biggest questions in climate science: Is human-caused climate change making El Niños worse?
A new study suggests the answer is yes, and the reason can be found in undersea corals: A 1,000-year-long record from corals in the Galapagos Islands off South America shows that El Niño is intensifying as the planet warms, according to a new study titled "Recent strengthening of eastern Pacific ENSO is unprecedented in the last millennium paleorecord," which was published Aug. 27 in the peer-reviewed journal Science.
"Our findings tell us that the big El Niño events of the last 40 years are not normal in the context of the last thousand years," said lead author Julia Cole, professor and chair of the University of Michigan department of earth
and environmental sciences, in a statement.
According to the study, El Niño events in the last four decades were stronger than any in the 1,000 years prior to about 1850, when humans began impacting the climate with greenhouse gases.
The findings say that climate change is already amplifying one of the world’s most important sources of extreme weather, with potentially growing risks to people around the planet.
"We believe global warming is supercharging El Niño, and if that's true, then we expect stronger climate extremes that will amplify ecological, infrastructural and human losses," Cole said. "No country has the resources to be fully protected from these impacts."
Understanding El Niño
El Niño is a natural climate phenomenon that, every few years, causes the tropical Pacific Ocean to become warmer than usual. This sets off a complex dance between the ocean and the atmosphere, which leads to weather impacts all over the world.
The current El Niño could reach "very strong" levels, the National Oceanic and Atmospheric Administration's Climate Prediction Center has said. "There is a 69% chance of a historic event that would exceed the strength of previous El Niño events dating back to 1950," the CPC said in a recent outlook.
Overall, the El Niño-Southern Oscillation (ENSO) pattern – which includes La Niña – is a leading source of year-to-year climate extremes and is linked to droughts, floods, wildfires, hurricanes, coral bleaching events and impacts on agriculture and human health.
Here in the U.S., during an El Niño, storm tracks over the nation shift southward, bringing more rain to the Desert Southwest and less to the Northwest. Similar shifts around the world lead to droughts and flooding that bring disease, crop failure, wildfires and other crises that impact people's health and well-being, Cole said.
In addition, during a strong El Niño, hurricanes and tropical storms tend to be more prolific in the Pacific Ocean.
"The question is not whether the current El Niño is going to happen, but how bad is it going to be, and how bad will the impacts be?" Cole said. "This event is superimposed on global warming, and it's likely to supercharge the temperature increase that we would normally see from greenhouse gases."
How can coral give us clues about past El Niños?
Corals grow in the regions where El Niños are strongest, and they can live for decades to centuries, according to Cole. They create durable skeletons of calcium carbonate, and the detailed chemistry of the skeleton tells us about the temperature at which they grew.
Specifically, corals grow one to two centimeters per year by secreting layers of calcium carbonate. The chemistry of these layers provides a record of the seawater temperature in which the corals grew.
How does climate change cause El Niños to be stronger?
Cole said that the mechanism by which warming changes El Niño strength is still a subject of intense research in the climate community, and that "our paper does not dig deeply into this. Several possible explanations have been put forward; one possible candidate has to do with how the atmosphere influences the ocean," Cole told USA TODAY.
As the tropical Pacific region warms and gets more rainy, the surface of the ocean gets warmer and fresher.
Both of those make the surface ocean less dense, and that lighter water is harder to mix down deeper – the layering of the ocean becomes stronger ("more stratified"), according to Cole. This makes the surface layer more sensitive to changes in the winds that play such an important role in triggering and maintaining El Niños.
Study gets mixed review from expert
The University of Pennsylvania's Michael Mann, a top climate expert, gave the study a mixed review:
"It’s an intriguing article, presenting interesting and important new coral data that provide potential insight into past El Niño variability. At face value, it would seem to suggest that recent ENSO variability is greater than what we see in the past from paleo data. But I’m not convinced the data actually show that," he told USA TODAY.
Mann said that the study doesn't have a complete record of corals over the past 1,000 years, and also that his research has shown that volcanic eruptions have played more of a role than climate change in past El Niño events.
He also said that it remains unclear whether the magnitude of El Niño events is changing. "There’s a lot of debate about that, and models tend to be split on whether or not we’re likely to see larger El Niños because of human-caused warming."
Mitigating global warming is key
Cole says her team's findings underscore the need to mitigate additional global warming. "El Niño is a really big source of climate extremes, and so if it's getting stronger, the impacts are getting stronger. Impacts like droughts, floods, wildfire and food insecurity," she said in a statement.
"Changes in the hydrologic cycle also lead to issues like damage to infrastructure: floods that wipe out homes, highways or railroads, or to health effects, such as diseases like cholera. This is one more reason we need to move away from fossil fuels, the root cause of the problem."
Doyle Rice is a national correspondent for USA TODAY, with a focus on weather and climate.
This article originally appeared on USA TODAY: El Niño is being supercharged by climate change, study finds
